1. Field of the Invention
This invention relates to storage arrangements for irradiated fuel following its removal from nuclear reactors. The invention can also relate to the storage of pre-irradiated fuel and also vitrified waste after spent fuel reprocessing.
2. Description of the Related Art
It is a common practice to store spent fuel under water, in what are generally known as pond stores, for periods that are long enough to allow the decay heat and radiation levels to reduce sufficiently to allow the fuel to be transported with safety. However, the use of a pond store is not entirely satisfactory where the fuel needs to be stored for any considerable length of time.
Thus, the ability to store the fuel safely for protracted periods in a water environment is very dependent upon the materials of the cladding in which the fuel is accommodated, the irradiation history of the fuel and/or the cladding, the integrity of the cladding, and the quality of the water in which the fuel is stored. Thus, cooling and shielding functions can be carried out completely satisfactorily while the fuel cladding remains intact, and while the water is present.
However, if the fuel cladding is perforated by corrosion or handling, then fission products can escape, and both fission products and corrosion products that are radioactively contaminated are then able to float and permeate to the surface of the water, which could result in high dose rates to operators. In addition, it is possible for these fission products and corrosion/contamination products to adhere to the walls of the pond. Variations in the pond water level, due to evaporation or leakage, could allow these products to dry out, when they could then become airborne, causing possible ingestion hazards to operators and the risk of atmospheric pollution.
Moreover, in order to maintain adequate cooling and shielding, the pond integrity must be assured to very high limits. Small leaks could give rise to minor contamination problems, and larger leaks, resulting in loss of cooling water, may result in a serious district hazard.
As safety requirements for nuclear installations become more rigorous, and the allowable dose rates to operators continue to decrease, the need to design storage systems and other nuclear installations to even higher orders of integrity becomes essential, particularly as for various reasons it is now becoming necessary to store spent nuclear fuel for longer periods than was originally anticipated.
In United Kingdom patent application No. 2061798, there is described and claimed an alternative form of storage arrangement which substantially avoids the above-mentioned disadvantages. Such a storage arrangement comprises an enclosure for the fuel that utilizes air as its storage medium; an exhaust system for exhausting this air from the enclosure through filters so as to maintain the interior of the enclosure at subatmospheric pressure; and a transfer mechanism for transferring fuel into and from the enclosure.
Maintaining a depression or underpressure within the enclosure could eliminate the need for a high integrity envelope for the enclosure, as any leakage that might occur will be into the enclosure and, accordingly, the invention provides an inherently safer store than the usual water filled pond. In addition, as the fuel is stored in air rather than water, the risk of corrosion is reduced, and consequently the need for an operator to maintain the water chemistry at precise levels in order to prevent the generation of corrosion products, and the possibility of atmospheric pollution is thereby avoided.
Another form of dry storage arrangement for irradiated nuclear fuel is described in United Kingdom Patent Specification No. 1583303, such an arrangement comprising a grid having a plurality of openings for supporting respective fuel cans so that they extend downwards therefrom, the space above the grid forming an-air filled enclosure associated with an exhaust system for exhausting air from the space through filters to maintain the interior of this space at subatmospheric pressure, and the arrangement including means for producing a flow of cooling air over the exterior of the cans. In use of such an arrangement, the fuel is first enclosed in cans, and the cans placed in openings in the grid, the unused openings being sealed with lids. However, the lids need to be removed for accommodating further cans, which is inconvenient, and failure of a can could give rise to contamination of the cooling air, with the risk of polluting the atmosphere.